Dozens of monoclonal cultures of small-celled araphid diatoms from brackish or marine habitats worldwide were analyzed using morphological observations (light and electron microscopy) and molecular data (nuclear-encoded small subunit ribosomal RNA and chloroplast encoded rbcL and psbC). As a result, we established one new genus Gedaniella, distinguished by a shared morphological character (occlusions of branched volae, projecting from the apical sides of the areolae) and some molecular data, including five new species: G. alfred-wegeneri, G. arenaria, G. boltonii, G. panicellus, G. paucistriata and three new combinations: G. flavovirens, G. guenter-grassii and G. mutabilis. Additionally we describe eleven novel species within the existing genus Serratifera: S. andersonii, S. brevis, S. clavata, S. corallina, S. namibica, S. nosybeana, S. parkii, S. punctata, S. rhombica, S. sourniae, S. takanoi and one new combination S. opephoroides. Furthermore, five new taxa or combinations were described within other genera: such as Cratericulifera crinigera, Nanofrustulum wachnickianum, Plagiostriata baltica, Pseudostaurosira madagascariensis and Stauroforma rinceana. Detailed descriptions for established species are also provided: Nanofrustulum shiloi, Opephora pacifica and Pseudostaurosira elliptica. This study strongly suggests that the complete biodiversity of small-celled araphids is still far from known, and many species currently placed in Opephora and Pseudostaurosira need to be further re-investigated.

The diatom records of two sediment cores taken in the SW part of the Gulf of Gdańsk indicate paleoenvironmental variability in the last three stages of formation of the Baltic Sea: the Mastogloia, the Littorina and the Post-Littorina. Diatom community compositions of the cores show the influence of several factors on past environmental conditions of the study area: (1) regional salinity changes, (2) changes of the Vistula River inflow, and (3) sedimentary processes connected with the Hel Peninsula development.

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The palaeoenvironmental changes of the south-western part of the Gulf of Gdańsk during the last 8,000 years, with reference to the stages of the Baltic Sea, were reconstructed. Diatom analyses of two cores taken from the shallower and deeper parts of the basin enabled the conclusion to be drawn that the microflora studied developed in the three Baltic phases: Mastogloia, Littorina and Post-Littorina. Moreover, the so-called anthropogenic assemblage was observed in subbottom sediments of the study area.

Diatom analyses have been performed on a 425cm long Holocene marine sediment sequence from the North Atlantic south of Iceland and a 920cm sediment core spanning the last c. 7500 years from Skalafjord, Faeroe Islands. Additional core data include stable isotopes and results from magnetic susceptibility measurements, while chronostratigraphic control has been provided by AMS 14C measurements. The diatom records reveal distinct changes in North Atlantic surface circulation and climate that have been correlated with published terrestrial and marine records. Initial Holocene warming is dated at 9900 14C years BP, and after a Preboreal cold spell enhanced North Atlantic Current activity and warming prevailed from 9600 to 8800 14C years BP. From 8800 to 8000 "4C years BP the advection of warm Atlantic water masses weakened, presumably under atmospheric circulation conditions characterized by a dominating negative NAO. Prior to the Holocene Climatic Optimum (6000-5000 14C years BP) distinct hydrographic gradients existed in the northern North Atlantic that were probably most pronounced during the reported '8200 cal. years BP cold event'. After 5000 "4C years BP both core records indicate increased climate instability and periods of cooling ('Neoglaciation'), with enhanced cyclone activity affecting the northern North Atlantic particularly at around 4700, 4200, 3200, 2000, 1500 and 1000 14C years BP.

The paper presents the history of diatom studies carried out in the Gulf of Gdańsk and the Vistula Lagoon since the beginning of the 20 th century. One aspect of the research focuses on describing the structure of planktonic and benthic communities living in different parts of the area or occurring in their surface sediments as taphocoenoses. Other investigations concentrate on the fossil diatom flora, which is widely used in the research on the Late Glacial and Holocene to resolve paleogeographic, sedimentological and paleoecological problems.

The aim of this study was to analyze resting spores of Chaetoceros spp. preserved in Middle and Late Holocene sediments of the three parts of the Gulf of Gdańsk, southern Baltic Sea. The material studied consists of ten cores retrieved from Kuźnica Deep (Puck Lagoon), as well as the shallower and deeper parts of the Outer Puck Bay region. Our results indicate that the abundance of Chaetoceros resting spores in diatom taphocoenoses corresponded to regional and local paleoecological changes registered in Mastogloia, Littorina and Post-Littorina sediments, and that they also reflect human impact in the last hundred years.

The paper presents the results of the analysis of diatoms from surface sediments (stones, sands) and macroflora (seagrass, macroalgae) collected at 16 sampling sites located along the inner coastal zone of Puck Bay (southern Baltic Sea) along the Hel Peninsula. The main diatom species of epilithon, epipsammon and epiphyton were characterized with respect to their autecological preferences (habitat, salinity, trophic status, saprobity). Three groups of diatoms were distinguished with respect to the type of substrate based on the results of benthic flora analysis: diatoms (i) of one type of substrate, (ii) of two types and (iii) those occurring on all types of substrates. Moreover, the distribution of benthic diatom communities indicates ecological differences in the study area. Marine and brackish-water species were observed in large numbers in the coastal zone of the Outer Puck Bay, whereas freshwater flora occurred with a higher frequency in the coastal zone of the Puck Lagoon. The content of polysaprobionts and of α-mesosaprobionts indicates that the region of the Hel Tip is highly eutrophicated and very polluted. The coast in the vicinity of Kuznica is less polluted, whereas the best environmental conditions are found in the Jurata–Jastarnia region, as evidenced by the frequency of diatoms that are β-mesosaprobionts.